An accelerometer is a sensor typically utilized for measuring acceleration forces. These forces may be static, like the constant force of gravity, or they can be dynamic, caused by moving or vibrating the accelerometer. An accelerometer may sense acceleration or other phenomena along one, two, or three axes or directions. From this information, the movement or orientation of the device in which the accelerometer is installed can be ascertained. Accelerometers are used in inertial guidance systems, in airbag deployment systems in vehicles, in protection systems for a variety of devices, and many other scientific and engineering systems.
Accelerometers can be broadly categorized on the principle used to detect acceleration. These categories generally include displacement sensing and force sensing. Displacement sensing accelerometers operate by transducing the acceleration to be measured into a displacement of movable mass. This displacement can then be picked up by capacitive, optical, piezoresistive, or tunneling techniques. Since displacement sensing accelerometers require deflection, typical overload performance can have non-linearity issues, such as non-linear capacitive function, hitting stops, and so forth. In addition, displacement sensing accelerometers can suffer from “stiction”, i.e., in-use sticking, and have limited sensing frequencies.
Accelerometers based on force sensing operate by directly detecting the force, or stress, applied on a proof mass. Resonant sensing of accelerations can be classified under the category of force sensing since the input acceleration is detected in terms of a shift in the resonant characteristics of a sensing device coupled to a proof mass. In one such accelerometer, one or more elongate beams are coupled between a fixed frame, or multiple fixed anchors, and a suspended proof mass. A force, which may be electrostatic, electromagnetic or piezoelectric, is applied to the beams to cause them to vibrate transversely at a resonant frequency. The accelerometer is designed so that acceleration force applied to the proof mass along a fixed axis will cause tension or compression of the beams, which changes the resonant frequency of the vibrating beams. The acceleration force applied to the proof mass can be quantified by measuring the resonant frequency of the beams.
Resonant force sensing accelerometers do not suffer from some of the problems of displacement sensing accelerometers, due in part because resonant accelerometers have very low deflections and high stiffness. In addition, resonant accelerometers have the ability to sense higher frequencies than that of, for example, capacitive displacement accelerometers.
However, a problem with the design of small, high performance resonant accelerometer sensors involves obtaining an accurate acceleration measurement in the presence of packaging-induced stress. More specifically, the conventional accelerometer design with beams extending between a fixed frame and a proof mass can impose forces from package stress on the order of one hundred to one hundred thousand times greater than the acceleration forces. Likewise, with resonant accelerometer designs that use multiple anchors at different locations, the relative locations of these multiple anchors change under conditions of elevated temperatures thereby resulting in inaccurate acceleration force measurements due to thermal stress. In addition, the operational temperature of an accelerometer changes, so that acceleration force measurements due to thermal stress is not constant. The package stress and operational temperature changes can cause significant acceleration measurement errors in resonant accelerometers.
Accordingly, what is needed is a small, high performance resonant accelerometer that is able to sense high frequencies, and in which packaging-induced stress has little effect on acceleration measurements.